Is not locking task_lock in cgroup_fork() safe?

Li Zefan lizefan at
Mon Oct 8 05:46:18 UTC 2012

On 2012/10/8 10:01, Tejun Heo wrote:
> On Mon, Oct 08, 2012 at 11:00:00AM +0900, Tejun Heo wrote:
>> 7e381b0eb1 ("cgroup: Drop task_lock(parent) on cgroup_fork()") removed
>> task_lock from cgroup_fork citing that current->cgroups can't change
>> due to threadgroup_change locking; however, threadgroup_change locking
>> is used only during CLONE_THREAD forking.  If @current is forking a
>> new process, there's nothing preventing someone else to migrate the
>> parent while forking is in progress and delete the css_set it
>> currently is using.  Am I confused somewhere?

You're right. threadgroup lock is held unconditionally in attach_task_py_pid(),
but it's held only for CLONE_THREAD in fork path, which I guess I overlooked
when reviewing the patch.

> Also, please note that task_lock is likely to be hot on local CPU at
> that point and avoiding it there might not really buy much.

Reverting that commit should be fine.

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